The invention concerns a device for increasing heat transmission to the cooling cylinders in rotary-offset machines.
Stock that has been printed in a rotary-offset machine is advanced through a drier that vaporizes or oxidizes the ink solvent. The ink can be dried with hot air or by gas flames that act directly on the web of stock. Once dry, the web travels over cooling cylinders that restore the ink and stock to normal temperature. Powerful cooling completely solidifies the ink, which will not smear during further processing.
High printing speeds impel the boundary layer of air adhering to the web and to the surface of the cooling cylinder into the area of the cylinder that is wrapped by the web, where it forms a cushion. This cushion of air considerably impedes the transmission of heat, and droplets of oil condense on the cylinder.
When the machine is operating slowly enough (approximately 5 m/sec) to maintain satisfactory contact between the web and the cooling cylinder, the oil can be removed from the web without sacrificing printing quality.
At a critical speed that depends on the overall geometry and on the specific tension on the web, however, the web will lift itself off the cooling cylinder and oil will become suspended in the air in the gap. Oil will accordingly accumulate on the first cooling cylinder, dissolving the dry ink again and smearing the print.
As the speed increases, the coefficient of heat transmission from the printed matter or web to the cooling cylinder will decrease steadily. Any increase in heat resistance will obviously depend on the thickness of the enclosed air gap.
A known device for eliminating re-vaporized oil is called a dynamic boundary-layer doctor. This component is positioned directly behind the drier and eliminates some of the re-vaporized oil between the drier and the first cooling cylinder by blowing air onto it from a circulating system that communicates with a condenser for separating the oil.
This device is very complicated and even so does not ensure high-quality printing because it cannot eliminate enough oil.
In what is called chill jetting, a narrow jet of air is directed at the web where the web comes into contact with the cylinder to force them together tighter.
But this procedure is also unsatisfactory at high printing speeds because it does not prevent oil condensation leading to deposits of ink on the cooling cylinder that powerfully deteriorate the printing quality.